Re: [PATCH RFC] NFSD: Fix possible sleep during nfsd4_release_lockowner()

From: Jeff Layton <jlayton@kernel.org>
To: Trond Myklebust <trondmy@hammerspace.com>,
	"chuck.lever@oracle.com" <chuck.lever@oracle.com>
Cc: "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH RFC] NFSD: Fix possible sleep during nfsd4_release_lockowner()
Date: Mon, 23 May 2022 14:04:39 -0400	[thread overview]
Message-ID: <a719ae7e8fb8b46f84b00b27d800330712486f40.camel@kernel.org> (raw)
In-Reply-To: <93d11e12532f5a10153d3702100271f70373bce6.camel@hammerspace.com>

On Mon, 2022-05-23 at 17:43 +0000, Trond Myklebust wrote:
> On Mon, 2022-05-23 at 12:37 -0400, Jeff Layton wrote:
> > On Mon, 2022-05-23 at 15:41 +0000, Chuck Lever III wrote:
> > > 
> > > > On May 23, 2022, at 11:26 AM, Jeff Layton <jlayton@kernel.org>
> > > > wrote:
> > > > 
> > > > On Mon, 2022-05-23 at 15:00 +0000, Chuck Lever III wrote:
> > > > > 
> > > > > > On May 23, 2022, at 9:40 AM, Jeff Layton <jlayton@kernel.org>
> > > > > > wrote:
> > > > > > 
> > > > > > On Sun, 2022-05-22 at 11:38 -0400, Chuck Lever wrote:
> > > > > > > nfsd4_release_lockowner() holds clp->cl_lock when it calls
> > > > > > > check_for_locks(). However, check_for_locks() calls
> > > > > > > nfsd_file_get()
> > > > > > > / nfsd_file_put() to access the backing inode's flc_posix
> > > > > > > list, and
> > > > > > > nfsd_file_put() can sleep if the inode was recently
> > > > > > > removed.
> > > > > > > 
> > > > > > 
> > > > > > It might be good to add a might_sleep() to nfsd_file_put?
> > > > > 
> > > > > I intend to include the patch you reviewed last week that
> > > > > adds the might_sleep(), as part of this series.
> > > > > 
> > > > > 
> > > > > > > Let's instead rely on the stateowner's reference count to
> > > > > > > gate
> > > > > > > whether the release is permitted. This should be a reliable
> > > > > > > indication of locks-in-use since file lock operations and
> > > > > > > ->lm_get_owner take appropriate references, which are
> > > > > > > released
> > > > > > > appropriately when file locks are removed.
> > > > > > > 
> > > > > > > Reported-by: Dai Ngo <dai.ngo@oracle.com>
> > > > > > > Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> > > > > > > Cc: stable@vger.kernel.org
> > > > > > > ---
> > > > > > > fs/nfsd/nfs4state.c |    9 +++------
> > > > > > > 1 file changed, 3 insertions(+), 6 deletions(-)
> > > > > > > 
> > > > > > > This might be a naive approach, but let's start with it.
> > > > > > > 
> > > > > > > This passes light testing, but it's not clear how much our
> > > > > > > existing
> > > > > > > fleet of tests exercises this area. I've locally built a
> > > > > > > couple of
> > > > > > > pynfs tests (one is based on the one Dai posted last week)
> > > > > > > and they
> > > > > > > pass too.
> > > > > > > 
> > > > > > > I don't believe that FREE_STATEID needs the same
> > > > > > > simplification.
> > > > > > > 
> > > > > > > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > > > > > > index a280256cbb03..b77894e668a4 100644
> > > > > > > --- a/fs/nfsd/nfs4state.c
> > > > > > > +++ b/fs/nfsd/nfs4state.c
> > > > > > > @@ -7559,12 +7559,9 @@ nfsd4_release_lockowner(struct
> > > > > > > svc_rqst *rqstp,
> > > > > > > 
> > > > > > >                 /* see if there are still any locks
> > > > > > > associated with it */
> > > > > > >                 lo = lockowner(sop);
> > > > > > > -               list_for_each_entry(stp, &sop->so_stateids,
> > > > > > > st_perstateowner) {
> > > > > > > -                       if (check_for_locks(stp-
> > > > > > > > st_stid.sc_file, lo)) {
> > > > > > > -                               status = nfserr_locks_held;
> > > > > > > -                               spin_unlock(&clp->cl_lock);
> > > > > > > -                               return status;
> > > > > > > -                       }
> > > > > > > +               if (atomic_read(&sop->so_count) > 1) {
> > > > > > > +                       spin_unlock(&clp->cl_lock);
> > > > > > > +                       return nfserr_locks_held;
> > > > > > >                 }
> > > > > > > 
> > > > > > >                 nfs4_get_stateowner(sop);
> > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > > lm_get_owner is called from locks_copy_conflock, so if
> > > > > > someone else
> > > > > > happens to be doing a LOCKT or F_GETLK call at the same time
> > > > > > that
> > > > > > RELEASE_LOCKOWNER gets called, then this may end up returning
> > > > > > an error
> > > > > > inappropriately.
> > > > > 
> > > > > IMO releasing the lockowner while it's being used for
> > > > > _anything_
> > > > > seems risky and surprising. If RELEASE_LOCKOWNER succeeds while
> > > > > the client is still using the lockowner for any reason, a
> > > > > subsequent error will occur if the client tries to use it
> > > > > again.
> > > > > Heck, I can see the server failing in mid-COMPOUND with this
> > > > > kind
> > > > > of race. Better I think to just leave the lockowner in place if
> > > > > there's any ambiguity.
> > > > > 
> > > > 
> > > > The problem here is not the client itself calling
> > > > RELEASE_LOCKOWNER
> > > > while it's still in use, but rather a different client altogether
> > > > calling LOCKT (or a local process does a F_GETLK) on an inode
> > > > where a
> > > > lock is held by a client. The LOCKT gets a reference to it (for
> > > > the
> > > > conflock), while the client that has the lockowner releases the
> > > > lock and
> > > > then the lockowner while the refcount is still high.
> > > > 
> > > > The race window for this is probably quite small, but I think
> > > > it's
> > > > theoretically possible. The point is that an elevated refcount on
> > > > the
> > > > lockowner doesn't necessarily mean that locks are actually being
> > > > held by
> > > > it.
> > > 
> > > Sure, I get that the lockowner's reference count is not 100%
> > > reliable. The question is whether it's good enough.
> > > 
> > > We are looking for a mechanism that can simply count the number
> > > of locks held by a lockowner. It sounds like you believe that
> > > lm_get_owner / put_owner might not be a reliable way to do
> > > that.
> > > 
> > > 
> > > > > The spec language does not say RELEASE_LOCKOWNER must not
> > > > > return
> > > > > LOCKS_HELD for other reasons, and it does say that there is no
> > > > > choice of using another NFSERR value (RFC 7530 Section 13.2).
> > > > > 
> > > > 
> > > > What recourse does the client have if this happens? It released
> > > > all of
> > > > its locks and tried to release the lockowner, but the server says
> > > > "locks
> > > > held". Should it just give up at that point? RELEASE_LOCKOWNER is
> > > > a sort
> > > > of a courtesy by the client, I suppose...
> > > 
> > > RELEASE_LOCKOWNER is a courtesy for the server. Most clients
> > > ignore the return code IIUC.
> > > 
> > > So the hazard caused by this race would be a small resource
> > > leak on the server that would go away once the client's lease
> > > was purged.
> > > 
> > > 
> > > > > > My guess is that that would be pretty hard to hit the
> > > > > > timing right, but not impossible.
> > > > > > 
> > > > > > What we may want to do is have the kernel do this check and
> > > > > > only if it
> > > > > > comes back >1 do the actual check for locks. That won't fix
> > > > > > the original
> > > > > > problem though.
> > > > > > 
> > > > > > In other places in nfsd, we've plumbed in a dispose_list head
> > > > > > and
> > > > > > deferred the sleeping functions until the spinlock can be
> > > > > > dropped. I
> > > > > > haven't looked closely at whether that's possible here, but
> > > > > > it may be a
> > > > > > more reliable approach.
> > > > > 
> > > > > That was proposed by Dai last week.
> > > > > 
> > > > > https://lore.kernel.org/linux-nfs/1653079929-18283-1-git-send-email-dai.ngo@oracle.com/T/#u
> > > > > 
> > > > > Trond pointed out that if two separate clients were releasing a
> > > > > lockowner on the same inode, there is nothing that protects the
> > > > > dispose_list, and it would get corrupted.
> > > > > 
> > > > > https://lore.kernel.org/linux-nfs/31E87CEF-C83D-4FA8-A774-F2C389011FCE@oracle.com/T/#mf1fc1ae0503815c0a36ae75a95086c3eff892614
> > > > > 
> > > > 
> > > > Yeah, that doesn't look like what's needed.
> > > > 
> > > > What I was going to suggest is a nfsd_file_put variant that takes
> > > > a
> > > > list_head. If the refcount goes to zero and the thing ends up
> > > > being
> > > > unhashed, then you put it on the dispose list rather than doing
> > > > the
> > > > blocking operations, and then clean it up later.
> > > 
> > > Trond doesn't like that approach; see the e-mail thread.
> > > 
> > 
> > I didn't see him saying that that would be wrong, per-se, but the
> > initial implementation was racy.
> > 
> > His suggestion was just to keep a counter in the lockowner of how
> > many
> > locks are associated with it. That seems like a good suggestion,
> > though
> > you'd probably need to add a parameter to lm_get_owner to indicate
> > whether you were adding a new lock or just doing a conflock copy.
> 
> I don't think this should be necessary. The posix_lock code doesn't
> ever use a struct file_lock that it hasn't allocated itself. We should
> always be calling conflock to copy from whatever struct file_lock that
> the caller passed as an argument.
> 
> IOW: the number of lm_get_owner and lm_put_owner calls should always be
> 100% balanced once all the locks belonging to a specific lock owner are
> removed.
> 

We take references to the owner when we go to add a lock record, or when
copying a conflicting lock. You want to keep a count of the former
without counting the latter.

lm_get_owner gets called for both though. I don't see how you can
disambiguate the two situations w/o some way to indicate that. Adding a
bool argument to lm_get_owner/lm_put_owner ops would be pretty simple to
implement, I think.

-- 
Jeff Layton <jlayton@kernel.org>